The present invention relates generally to a cryogenic cooling system for synchronous machine having a rotor with a high temperature superconducting (HTS) component. More particularly, the present invention relates to a cooling system to provide cryogenic fluid to the rotor of an HTS machine, and to re-cool used cooling fluid returned from the rotor.
Superconducting rotors have their superconducting coils cooled by liquid helium, with the used helium being returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room temperature gaseous helium. Thus, reliquefaction poses significant reliability problems and requires significant auxiliary power for the helium cooling system. Accordingly, there is a need for a cryogenic cooling system that reliquefies the hot, used cooling fluid returned from the rotor. The reliquefied cooling fluid should then be available for re-use to continuously cool the HTS rotor.
A cryorefrigeration system for a superconducting electric machine provides cooling fluids to cool to cryogenic temperatures, and to maintain the superconducting characteristics of components of the machine, e.g., superconducting rotor coils. For economic reasons, electric machines are expected to have high availability and reliability. However, some of the cryorefrigeration components, such as a coldhead and compressors in a Gifford-McMahon (GM) refrigeration system, have short operational life spans due to wear and experience cooling fluid leakage as result of reciprocating mechanical motion of the components of the cooling system. The reliability of the cold head and Gifford-McMahon systems may not be sufficient for the high reliability requirements of certain machines, especially for industrial power machines such as power generators.
High temperature superconducting generators require highly reliable, low cost cryorefrigeration equipment to be viable as commercial products. Redundant cryorefrigerator components have in the past been used to achieve high reliability with existing cryorefrigeration equipment. The inadequate reliability of individual cooling components and the requirement that HTS rotors have an uninterrupted supply of cooling fluid have in the past necessitated that redundant components be included in cryorefrigeration systems for HTS rotors.
Cooling systems must continuously operate in a machine having superconducting components. When a cryogenic cooling component fails, cooling fluid may not properly flow to the machine and superconducting components in the machine warm up. These warmed components lose their superconducting characteristics and the machine operation is interrupted due to loss of superconductivity. Accordingly, cooling systems with inadequate reliability will cause machine shut-downs that are unexpected and undesirable. To maintain system availability, a fully redundant cooling system path has typically been used in the past to improve the reliability of the cooling system. Due to redundant system components, the cost of the cryorefrigeration systems for superconducting machines is nearly doubled. Moreover, existing cryorefrigeration systems require frequent maintenance due to their inadequate reliability and system redundancies. Accordingly, the capital and operating cost of these cryogenic cooling systems is relatively high.
Typical cryorefrigerator equipment for the temperature range of 20-30xc2x0 Kelvin (K) is based on Gifford McMahon cold head technology that has limited refrigerator capacity and requires maintenance about once a year. Multiple units can be combined to increase the capacity and reliability of the system at the expense of increased cost. In addition to multiple (redundant) cold heads, closed loop circulation systems require either cold re-circulation fans, or external warm re-circulation fans with counter-flow highly efficient heat exchangers. These components add cost and complexity to the system when redundancy for high reliability is required, unless all components can be built with six sigma quality.
The purchase and operating costs of existing cryorefrigeration systems significantly adds to the cost of machines having HTS rotors. These high costs have contributed to the commercial impracticalities of incorporating HTS rotors into commercially marketable synchronous machines. Accordingly, there is a substantial and previously unmet need for cryorefrigeration systems that are less expensive, inexpensive to operate and provide a reliable supply of cryogenic cooling fluid to a HTS rotor.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron forging.
A short-term temporary cooling system for a superconducting machine is disclosed. The temporary cooling system operates during maintenance or a failure of a main cooling system component, and until the main cooling system can be serviced and be made operational. The temporary cooling system has a lower initial cost and lower lifetime operational cost as compared to the costs of a conventional redundant cooling path system.
The temporary cooling system may provide several hours of cryogenic cooling of the cooling fluid passing through the SC machine. These hours of cooling while the main cooling system is at least partially inoperative allow for continued operation of the superconducting machine. While the temporary cooling system operates, an established superconducting machine service network should be able to service the failed main cooling system components and resort the operation of the main cooling system (and then turn-off the temporary cooling system). Accordingly, the temporary cooling system provides continued cryogenic cooling fluid for a SC machine for a defined time period, such as several hours.
In a first embodiment, the invention is a cooling fluid system for providing cryogenic cooling fluid to a high temperature superconducting machine, wherein said system includes a main cooling system (52, 88) and a second cooling system, said second cooling system comprising: a storage device having a first cryogenic fluid; at least one cooling coupling in fluid communication with the first cryogenic fluid from the storage device and a second cryogenic fluid flowing through the main cooling system, and said second cooling system has a first operational mode during which the first cryogenic fluid does not flow through the at least one cooling coupling, and a second operational mode during which the first cryogenic fluid does flow through the at least one cooling coupling, wherein said second cooling system is switched from the first operational mode to the second operational mode when a failure occurs in the main cooling system.
In another embodiment, the invention is a cooling fluid system coupled to a high temperature superconducting rotor for a synchronous machine, said system comprising: a main cooling system that further comprises a re-circulation compressor; an inlet line providing a fluid passage for a second cooling fluid flowing from the re-circulation compressor to the rotor, wherein the inlet line passes through a cold head unit; and a temporary cooling system that further comprises: a storage tank for a first cryogen fluid; at least one heat exchanger in fluid communication with said storage tank and said inlet line, and a valve between the tank and at least one heat exchanger, said valve having an open position allowing the first cryogen fluid to flow from the tank to the at least one heat exchanger and a closed position isolating the first cryogen fluid from the at least one heat exchanger, wherein the open position of the valve is selected when the cold head unit is disabled and said closed position is selected when the cold head unit is operating to cool the second cryogen cooling fluid.
In a further embodiment, the invention is a method for cooling a super-conducting machine using a main cooling system and a temporary cooling system, comprising the steps of: storing a first cryogenic cooling fluid in said temporary cooling system; circulating a second cryogenic cooling fluid between said main cooling system and said machine to cryogenically cool superconducting components of the machine; cooling the second cryogenic cooling fluid with a cooling unit in said main cooling system and at the same time thermally isolating the first cryogenic cooling fluid from said second cryogenic cooling fluid; while said cooling unit is disabled, allowing the first cryogenic cooling fluid to cryogenically cool the second cooling fluid.